1. Field of the Invention
This invention relates to a high-speed transmission cable conductor using a core material made of copper or a copper alloy, and a producing method therefor, and a high-speed transmission cable.
2. Description of the Related Art
Electronic devices such as servers, routers, storages, etc. handle high-speed digital signals with a transmission speed of several Gbps or higher. The electronic devices of this kind require interfaces for lessening signal waveform degradation and having excellent high-frequency transmission properties, in signal transmissions between devices, between chassis in devices, between substrates in devices, and the like. One of the interfaces is a high-speed transmission cable.
This high-speed transmission cable generally uses a coaxial cable. The coaxial cable comprises a core wire (also called inner conductor or center conductor), an insulator which covers a circumference of the core wire, an outer conductor which covers a circumference of the insulator, and a jacket (also called sheath) which covers a circumference of the outer conductor.
The core wire is connected with a signal line of the electronic devices, while the outer conductor is connected with ground of the electronic devices, so that a signal is transmitted in the core wire. In the case where a plated wire, in which a material having high resistance is arranged as its surface layer, is used in the coaxial cable, when the frequency of the signal to be transmitted is high (i.e., the transmission rate is high), transmission loss is significant due to a skin effect caused in the core wire. This tendency manifests more prominently in accordance with the increase in cable length. As a result, long-distance signal transmission causes the rounding of the originally rectangular digital signal waveform. The transmitted signal degrades and cannot properly be transmitted.
In the case where a tin-plated copper wire to be used in a general cable is used in the high-speed transmission cable, due to the Sn having a significantly low electrical conductivity of 15% IACS plated as the conductor surface layer around the annealed copper having an electrical conductivity of 100% IACS, the skin effect increases the attenuation of the transmitted signal caused by the resistance of the conductor in a high frequency region.
Therefore, a conductor formed with Ag plating having high electrical conductivity around its surface has been selected as the conductor for the high-speed transmission cable for use in a high frequency region (see, e.g. JP-A-2006-307277 and JP-A-2008-293894).
In the high-speed transmission cable conductor disclosed in JP-A-2006-307277, in order to reduce the occurrence of wire breaking, a wire rod made of copper or a copper alloy has been formed with a Ag plating layer therearound which is harder than that wire rod. In the high-speed transmission cable conductor disclosed in JP-A-2008-293894, in order to increase its bending resistance than a bending resistance of pure copper, a coating layer comprising Ag or Ag alloy has been formed around a circumference of a core material made of pure copper or a copper alloy.